(a) Field of the Invention
The present invention relates to a lighting apparatus and a liquid crystal display (LCD) device, and more particularly, to the specific structure of a lighting apparatus and an LCD device including such a lighting apparatus as a backlight unit.
(b) Description of the Related Art
An LCD device known as a transmissive LCD device includes a backlight unit as a lighting apparatus for irradiating an LC panel of the LCD device from the rear side of the LC panel. The backlight unit is generally categorized into two types: direct-emission type and an edge-emission type. The direct-emission type includes a plurality of lamps at the rear of the emission surface of the backlight unit, whereas the edge-emission type includes a light conductive plate on the rear side of the emission surface of the backlight unit for guiding light emitted by a lamp. The direct-emission type achieves a higher luminance compared to the edge-emission type, and thus is especially suited to a large-screen LCD device.
FIG. 21 shows a direct-emission backlight unit for use in an LCD device in an exploded perspective view thereof. FIG. 22 shows a sectional view of the backlight unit of FIG. 21, taken along line XXII-XXII in FIG. 21. The backlight unit, generally designated by numeral 60, includes a rear reflection plate 11, and a lamp assembly 12 including a plurality of elongate lamps 13 extending parallel to one another in front of the rear reflection plate 11. The lamp assembly 12 includes an inverter 14 connected at one ends of the lamps 13, a return substrate 15 connected to the other ends of the lamps 13, and a return cable 16 having a first end connected to the return substrate 15 and a second end connected to the ground of the inverter 14. The inner surface of the rear reflection plate 11 has a reflectance of 95%, for example, and has a light scattering property.
A pair of support members 17 support both ends of the elongate lamps 13 while receiving the same at the cutout portions 17a of the support members 17. A diffusion plate 18 and an optical sheet assembly 19 are consecutively disposed to press the support members 17 toward the lamps 13. The optical sheet assembly 19 may include a diffusion sheet, a lens sheet and a luminescent film, for example. The optical sheet assembly 19 and the diffusion plate 18 are received in a backlight chassis 20 having a front opening, which exposes therethrough the emission surface 20a of the backlight unit 60. The backlight unit 60 emits light through the emission surface 20a toward the LC panel not shown.
The distance between the diffusion plate 18 and the centers of the lamps 13 is about 10 mm, for example, whereas the center-to-center distance between two adjacent lamps 13 is about 20 mm, for example. Some light beams emitted from one of the lamps 13 are directly incident onto the diffusion plate 18, whereas other some light beams emitted from the one of the lamps 13 are incident onto the diffusion plate 18 after being reflected by the rear reflection plate 11. The light beams incident onto the diffusion plate 18 is diffused thereby and emitted outside the backlight unit 60 through the emission surface 20a. 
In an LCD device, the backlight unit is desired to have an emission surface having a higher luminance uniformity for achieving a superior image quality. However, the direct-emission backlight unit generally has an ununiform luminance profile on the emission surface thereof, as shown in FIG. 23, due to the arrangement of the lamps 13. The ununiform luminance profile is such that portions of the emission surface corresponding to the centers of the lamps have a higher luminance whereas the other portions of the emission surface corresponding to the gap between the lamps has a lower luminance. In a recent tendency, since the backlight unit has a smaller thickness due to the smaller thickness of the LCD device, the problem of the ununiform luminance profile on the emission surface is emphasized by a reduced distance between the emission surface and the lamps.
For suppressing the ununiform luminance on the emission surface of the backlight unit, a dot-pattern printing technique, for example, may be employed on the rear surface of the diffusion plate. The dot-pattern printing technique may be used so that the density of dots is highest at the fronts of the centers of the lamps, reduced along the lateral direction from the centers, and lowest at the fronts of the gap centers between the lamps. The dot pattern scatters the light emitted from the lamps to moderately reduce the luminance at the gaps between the lamps. The dot-pattern printing technique, however, involves a problem of a chromaticity difference on the emission surface due to the reflection characteristic of the ink material used for the printing.
Patent Publications JP-A-4(1992)-275525 and -11(1999)-126505, for example, describe techniques for suppressing the ununiformity of the luminance without using the dot-pattern printing technique. JP-A-4-275525 uses a rear reflection plate having a convex portion at the rear of the gap between the lamps, the convex portion having a higher reflectance to thereby increase the luminance at the front of the gap between the lamps. JP-A-11-126505 uses another rear reflection plate having convex portions and concave portions alternately arranged at a pitch corresponding to the pitch of the lamps, the concave portions being disposed corresponding to the centers of the lamps. The backlight units described in these publications may increase the luminance at the front of the gaps between the lamps, to thereby suppress the ununiformity of the luminance on the emission surface.
In the techniques described in the publications, there is a problem however that the optimum profile of the rear reflection plate differs depending on the distance between the diffusion plate and the lamps, pitch of the lamps, diameters of the lamps etc., which fact consumes a longer time in the design and production of the prototype backlight units for assuring the effectiveness of the design, although the design itself may be performed using a simulation.